Following [1]} and making the usual assumption that the frequencies of interest are located around \(\omega _{q}\) , one can exchange \(\sqrt{\omega _{k}}\)  with \(\sqrt{\omega _{q}}\)  and take it out of the sum. In this approximation, it turns out that \(f_{k}(\omega _{k},t)\)  defines the time-dependent wavepacket, i.e field amplitudes for a certain \(\omega _{k}\) . After exchanging the sum in expression REF  with an integral, one arrives at a usual truncated exponential decay.
